This invention relates generally to ink-jet printhead driver circuits and more particularly to bipolar integrated driver circuits.
Non-impact ink-jet printers are commonly known in the art. Ink-jet printers generate thermal energy, typically by passing a current through a resistive element, which causes an ink droplet to form. The ink droplet is ejected from an orifice, or nozzle, onto a predetermined position on a print media. A plurality of such ink drops are deposited on the print media to form a desired image. The construction of ink-jet printheads designed to accomplish this printing task is known in the art. Such ink-jet printheads consist of an ink-reservoir, the ink-nozzles, and the accompanying drive circuitry, including resistive elements.
Semiconductor drive circuits, which typically consist of a transistor or diode coupled to a thin-film resistor, are used to switch the thermal producing current to the desired resistor. FIG. 12 shows an NPN drive transistor, indicated generally at 920, formed on substrate 901, as is known in the art. Deposited on the substrate 901, over insulating layer 906, is a conductive layer 908 typically formed of Aluminum. A resistive layer 907 is formed of hafnium boride to act as a resistive element. A further insulating layer 909 is deposited over the conductive layer to insulate the Aluminum from the highly corrosive ink present in ink-passage 950, defined by top-plate 910 and ink-jet driver substrate 930.
The aforementioned drive circuit, however, suffers from well-known deficiencies, such as speed, cost, reliability, silicon area, and energy consumption. Several improved circuits have been designed which improve upon the basic design shown in FIG. 12. One such improved circuit is described in European Patent Application No. 91301019.5 by Matsumoto et al. for an Ink Jet Recording System. Referring to FIG. 13, a diode matrix drive circuit is shown as described by Matsumoto et al. By using a diode matrix for decoding, fewer interconnects are required, which produces a corresponding reduction in the silicon area required by the circuit.
Each of the diodes shown in FIG. 13 is constructed by forming an NPN drive transistor having a common base-collector electrode, as is known in the art. Referring now to FIG. 14, two such drive transistors SH1 and SH2 are shown. Drive transistor SH1 is formed on P- substrate 952, consisting of N type collector regions 954, 956, and 958, P type base regions 960 and 962, and an N+ type emitter region 964. Coupled to the base and collector regions is a common base-collector electrode 966, and coupled to the emitter region is electrode 968. A voltage supply VH is coupled to the common base-collector electrode 966 through an external pass transistor. The emitter electrode 968 is coupled to a resistive element RH1, which is further coupled to a common voltage supply through another external pass transistor. Drive transistor SH2 is similarly constructed.
Located between drive transistors SH1 and SH2 is a P type isolation region 970, which is connected to isolation electrode 974 through a heavily doped P-type isolation region 972. The P-type isolation region acts as an isolation domain, to minimize parasitic currents, which compromise the drive capability of the circuit. The isolation region, however, consumes valuable silicon area on the circuit.
Furthermore, the diode matrix design requires relatively low voltages to avoid exceeding the emitter-base breakdown voltage ("BV.sub.EB ") of the transistors. Exceeding the BV.sub.EB of the transistors can result in degradation of the transistor beta (".beta.") and perhaps make the transistor inoperable. In most processes, the emitter-base breakdown voltage is approximately 6-8 volts. Thus, relatively low voltages must be used. In order to produce the desired thermal dissipative energy, the diode matrix drivers typically require higher current levels, which results in higher power dissipation and lower long-term reliability.
Accordingly, a need remains for an integrated ink-jet printhead driver circuit which does not require an isolation region between adjacent drive transistors so as to minimize the silicon area of the drive circuit and which, further, uses lower current levels.